Ensure input-provider elements of collections merge inputs correctly

As reported in #78, if you compose an input filter provider fieldset as
a target element of a collection in order to allow providing a default
fieldset for the collection, input provider elements composed by the
input filter provider fieldset were not being merged correctly into the
input filter.

This patch checks to see if the fieldset's input filter represents a
collection, and, if so, will check its own composed input filter for the
element and merge it if found.